The present invention relates to improvements in detection systems of the passive infrared variety. More particularly, it relates to improvements in reflective optical systems of the type used in such detection systems to provide multiple zones of detection.
Heretofore, a variety of detection systems have been proposed and utilized for sensing the presence of an animate object in a region under surveillance. Such systems include conventional intruder detection systems, as well as those systems commonly found in commercial establishments that function, for example, to open doors for approaching pedestrians. Most popular of these systems at the present time are those that sense the presence of such objects from the infrared radiation (i.e., body heat) they give off. Such "passive infrared" detection systems usually comprise an infrared (IR) radiation-sensitive detector capable of producing an output signal in response to slight changes in the level (or rate of change) of IR radiation incident thereon, and a multi-element optical system for focusing the IR radiation emanating in several different detection zones (defined by the optical system) onto the detector. The detector commonly takes the form of a pair of closely spaced pyroelectric sensors which are connected in series opposition to provide common mode rejection of non-target-related signals. The detector output, after suitable signal processing (e.g., to discriminate against false-alarm-producing spurious sources) is used to produce a control signal which, as indicated above, can be used to sound an alarm, open a door, etc.
Referring to FIG. 1, passive IR detection systems are commonly contained in a small housing H which is adapted to be mounted on a wall W several feet (e.g., 5-10 feet) above floor or ground level L. The multi-element optical system used to concentrate IR radiation on the system's detector element often comprises a focusing mirror or parabolic shape (to avoid spherical abberations), and a plurality of planar mirrors which are arranged at different angels relative to the axis of the focusing mirror. This arrangement provides the detector with different fields of view which, in FIG. 1, define the zones of detection Z1-25. An example of such an optical system is disclosed in U.S. Pat. No. 4,258,255. As illustrated in FIG. 1, each detection zone has a specific detection range associated with it, the most distant targets being detectable in zone Z1, and the closest targets, perhaps only a few feet away, being detectable in zone 25.
Optical systems of the above type may be characterized as "single focal length" systems in that all rays incident on the focusing element, i.e., the parabolic mirror, travel about the same distance to the focal point of the parabola. Note, as will be apparent from the ensuing description of the invention, this does not necessarily result from the shape of the parabolic mirror, but rather from the fact that such systems use only a relatively small, axially-located portion of the parabola. Typically, the f/number of optical systems of the above type is approximately unity or greater. As a result of using a single focal length optical system, the size of an image formed on the detector will depend on the target distance from the detector. Referring to FIG. 1, it will be appreciated that a single focal length lens or mirror will produce smaller images of targets located in zone 25 than it will of the same size targets located in zone Z1. Thus, for a given target size moving at a constant rate, target images will vary in size, depending on the displacement between the target and detector. This variation in target image size is undesirable in that it gives rise to system sensitivity variations from zone-to-zone. Moreover, it increases the required amplifier bandwidth for target-related signals, thereby increasing the chance for false alarming.
One solution to the above-noted problems produced by target size variations from zone-to-zone is to use an optical system having several different focusing mirrors, each having a focal length related to the range of protection it, at least in part, defines. In U.S. Pat. No. 4,339,748, there is disclosed a passive IR intruder detection system employing a mirror assembly comprising a plurality of spherical mirror segments arranged in two or more ranks, each rank corresponding to a different operating range. The respective focal lengths of the mirror segments in the same rank are identical, but the focal lengths differ from rank-to-rank, those mirror segments corresponding to a longer operating range having a longer focal length than those segments corresponding to a shorter range. While this multi-focal length optical system results in target images of somewhat uniform size for all zones of protection, it does so at the high cost of using multiple focusing mirrors in a more complex optical assembly.